Opus Dei's chief activity is providing spiritual formation to people through retreats, recollections, prayer and study groups, workshops, classes and spiritual direction. One of the things Opus Dei emphasizes in this spiritual formation is the importance of addressing social needs. So Opus Dei's members often join with each other and others to undertake social initiatives. In the case of corporate works, the organizers of an initiative have entrusted the spiritual components of the initiative to Opus Dei, and Opus Dei in turn ensures the Christian orientation of the undertaking.

From the Opus Dei Information Handbook (more technical).

Opus Dei's main activity consists in offering its members, and other people, the spiritual means they need to live as good Christians in the midst of the world. With this formation, the individual members carry out their apostolic activity, bearing witness to Jesus Christ in their own surroundings.

You can't have charity without justice, solidarity, family and social responsibility, poverty, joy, chastity, friendship. Personal testimony is always the most important apostolate in Opus Dei. It is an apostolate of witness, of specific and effective help given to others, at work and in the other circumstances of daily life: a personal apostolate carried out through word and example.

For the faithful of Opus Dei, apostolic work with young students and workers is particularly important. Members dedicate such time and energy towards this apostolate as circumstances permit. The formation offered to young people emphasizes the need for growth in the spiritual life and the practice of the human virtues in their work and in their service to others. It helps each one to develop his or her own personality, without creating any sort of closed or exclusive group.

The faithful of the prelature and the cooperators, together with many other Catholics and non-Catholics throughout the world, set up educational, charitable and cultural undertakings, with the clear aims of providing formation and of rendering a service to society. These include schools, hospitals, universities, centers for professional formation, and the like. The members of Opus Dei promote and sustain ventures that respond to the real needs of their country or environment, taking it upon themselves to find the necessary funding.

The prelature can assume responsibility for ensuring the Christian spirit of an undertaking by providing doctrinal guidance and pastoral care. A formal agreement to this effect is established with those in charge, in accordance with the statutes of the undertaking. All ventures of this kind fully respect the freedom of individual consciences and are open to people of all creeds, races, and social conditions. The prelature does not involve itself in any profit-making, commercial or political venture.

The Opus Dei Prelature can enter into various types of agreements with apostolic activities:

(a) in the case of the corporate works of apostolate, Opus Dei morally guarantees the Christian orientation of the activities they provide;

(b) in other cases, Opus Dei provides spiritual help, but without officially giving any moral guarantee as regards the formation offered. This spiritual assistance can take a variety of forms, such as priestly ministry or religion classes.

Such agreements with the prelature do not modify the civil nature of these entities in any way. Responsibility for their functioning and government always rests with their directors and not with the Opus Dei Prelature.